1. Field of the Invention
The present invention generally relates to heat treatment of a semiconductor in semiconductor processing. The present invention particularly relates to a method of performing heat treatment of a semiconductor and to a susceptor used in the method.
2. Description of the Prior Art
Conventionally, infrared radiation heating and high-frequency induction heating have been the main current in the method of heating a semiconductor wafer in semiconductor processing, for example, epitaxial growth processing, or the like.
The infrared radiation heating is a typical one of the so-called hot wall heating methods in which not only a semiconductor wafer but a reaction vessel or the like is simultaneously heated. The hot wall heating method has a defect that it requires a reflector for preventing heat radiation out of the reaction vessel, means for cooling the reflector, and so on.
The high-frequency induction heating is carried out in such a manner than a susceptor which is supporting a semiconductor wafer thereon is heated by an induction current owing to high-frequency energy so that the semiconductor wafer is heated by the heat applied from the heated susceptor through transmission and radiation. The high-frequency induction heating is a typical one of the so-called cold wall type heating methods in which a reaction vessel and so on are not heated.
FIG. 7 shows an example of a horizontal epitaxial growth apparatus employing the conventional high-frequency induction heating method.
A susceptor C for supporting a semiconductor wafer B to be treated is disposed in a quartz pipe A for isolating a treatment atmosphere in the quartz pipe A from the external atmosphere, and a coil D is wound around the outside of the quartz pipe A coaxially with the later.
The susceptor C is constituted by a conductive material of carbon with its surface coated with silicon carbide. The susceptor C is heated by an induction current owing to a high-frequency magnetic field generated by the coil D, and the generated heat is applied to the semiconductor wafer B through transmission and radiation to thereby heat the semiconductor wafer B.
In the semiconductor heating apparatus used in semiconductor processing, it is required to generate a high temperature of 500.degree. through 1200.degree. C. and to isothermally keep the high temperature for hours, and therefore a very large amount of electric power is consumed.
In this regard, the cold wall heating can be easily carried out by means of the above-mentioned high-frequency induction heating, however, the high-frequency heating is disadvantageous in that the energy conversion efficiency is very low such that only about 10% of the consumed electric power is converted into heat required for heating.
In the high-frequency induction heating method, as well known, the heat energy generated in the susceptor C can be expressed as a function having main elements such as the magnetic field strength applied to the susceptor C, the frequency of change in direction of the applied magnetic field, the resistivity of the susceptor C, the permeability of the magnetic path of the coil D, and so on. In order to increase the generated heat energy, there are a method in which the resistivity of the susceptor is made smaller or the permeability of the magnetic path is made higher to thereby make higher the efficiency of conversion from the consumed electric power to the generated energy, and a method in which the strength of the magnetic field is made larger or the frequency of the alternating magnetic field is made higher to thereby make higher the efficiency of conversion from the consumed electric power to the generated energy.
Of those elements making the conversion efficiency higher, any materials available for the conductive susceptor C, except conventionally used carbon or the like, cannot be expected to provide remarkably low resistance and therefore it is impossible to make low the resistivity of the susceptor C.
The permeability of the magnetic path of the coil D, on the other hand, is limited to the permeability of a vacuum because the coil D is provided with an air core. In the existing circumstances, accordingly, there is not any other measure than making the magnetic field strength larger and the frequency of the alternating magnetic field higher to increase the generation of thermal energy by varying those elements in the conventional high-frequency induction heating system.
In the conventional high-frequency induction heating system, however, the electric power is supplied from an ordinary commercial AC power source E to the coil D through conversion means, such as a frequency converter F, an impedance matching circuitry G, etc., so that the magnetic field strength applied to the susceptor C is determined substantially in accordance with the current caused to flow in the coil D.
However, this current is so large that the loss due to the resistance of the coil D becomes large and therefore the coil per se must be cooled by water or the like, resulting in a limit in increase in the magnetic field strength.
Further, there are limits in frequency characteristics, control power, etc., of switching elements or the like used in the large power frequency converter F, and therefore it is impossible to expect much increase both in the frequency and in the control power.
In the impedance matching circuitry G, the coil current is so large that the copper loss becomes large and therefore the conversion efficiency is low.
Recently, the size of semiconductor wafers has been changed from the conventional values of 7.6 through 12.7 cm (3 through 5 inches) to 15.2 through 20.3 cm (6 through 8 inches). Also in this regard, heat treatment furnaces are desired to be improved in increase of the generated heat energy.